In our prior U.S. Patents, Halley, Polishing Pad With Built-In Optical Sensor, U.S. Pat. No. 6,986,701 (Jan. 17, 2006) and Wolf, Polishing Pad With Built-In Optical Sensor, U.S. Pat. No. 6,485,354 (Nov. 26, 2002), we described polishing pads with built-in optical sensor assemblies for use in chemical mechanical planarization processes. The sensors assemblies included light sources and light detectors which are used to detect changes in the surface of an integrated circuit or in the thickness of films built on the wafer during polishing so that technicians involved in polishing wafers know exactly when to stop polishing. The sensor assemblies and optical sensors are embedded within the thickness of the polishing pad, and thus eliminate the need for drilling holes into the platen, as we had previously proposed in Truer, Optical View Port For Chemical Mechanical Planarization Endpoint Detection, U.S. Pat. No. 6,146,242 (Nov. 14, 2000). The incorporation of the optical sensor into the pad also eliminated the need for drilling holes through the platen as suggested in Birang, Forming A Transparent Window In A Polishing Pad For A Chemical Mechanical Polishing Apparatus, U.S. Pat. No. 5,893,796 (Apr. 13, 1999). In Birang's system, an optical sensor was placed under the platen, which required that a hole be drilled in completely through the platen, and also permitted sensing only when the hole in the plated passed over the sensor. The wafers to be polished are composite structures that include strata of different materials. Typically, the outermost stratum is polished away until its interface with an underlying stratum has been reached. At that point it is said that the end point of the polishing operation has been reached. The optical emitter/detector embedded in the pad is able to detect transitions from an oxide layer to a silicon layer as well as transitions from a metal to an oxide, or other material. The optical sensor can also be used to measure the thickness of a layer in processes intended to planarize a layer while leaving a uniformly thick film on the wafer. When a system controller (any suitable computer and appropriate interfaces to the emitter/detector) determines that the polishing process has reached a desired endpoint (complete removal of a metal layer, or reduction of a layer to a desired thickness), the system controller provides output to an operator indicating that the process endpoint has been reached, or operates the associated polishing system to cease polishing. The real-time data derived from the optical sensor enables the end-point detection without the need to disengage the wafer from the polishing equipment and interrupt the polishing process. This greatly increases the efficiency of the polishing process.